1. Field of the Invention
The present invention relates to a fluidized bed product discharqe process and, more particularly to a process for the post reaction treatment of resins produced in gas phase reactors.
2. Description of the Prior Art
The introduction of high activity Ziegler-Natta catalyst systems has lead to the development of new polymerization processes based on gas phase reactors such as disclosed in U.S. Pat. No. 4,482,687 issued Nov. 13, 1984. These processes offer many advantages over bulk monomer slurry processes or solvent processes. They are more economical and inherently safer in that they eliminate the need to handle and recover large quantities of solvent while advantageously providing low pressure process operation.
The versatility of the gas phase fluid bed reactor has contributed to its rapid acceptance. Alpha-olefins polymers produced in this type of reactor cover a wide range of density, molecular weight distribution and melt indexes. In fact new and better products have been synthesized in gas phase reactors because of the flexibility and adaptability of the gas phase reactor to a large spectrum of operating conditions.
Unfortunately, the versatility of the resin post-reaction treatment process has not evolved at the pace of the gas phase reactor technology which has been extended to the production of fluidizable but non-free flowing solid polymer as disclosed in U.S. Pat. No. 4,710,538 issued Dec. 1, 1987. These non-free flowing granular resins are referred to sometimes as "sticky polymers" because of their tendency to aggregate in larger particles and to eventually form large chunks of solid polymer.
The term "sticky polymer" is defined as a polymer, which, although particulate at temperatures below the sticking temperature, agglomerates at temperatures above the sticking temperature. The term "sticking temperature", which, in the context of this specification, concerns the sticking temperature of particles of polymer in a fluidized bed, is defined as the temperature at which fluidization ceases due to the agglomeration of particles in the bed. The agglomeration may be spontaneous or occur on short periods of settling.
A polymer may be inherently sticky due to its chemical or mechanical properties or pass through a sticky phase during the production cycle. Sticky polymers are also referred to as non-free flowing polymers because of their tendency to compact into aggregates of much larger size than the original particles and not flow out of the relatively small openings in the bottom of product discharge tanks or purge bins. Polymers of this type show acceptable fluidity in a gas phase fluidized bed reactor; however, once motion ceases, the additional mechanical force provided by the fluidizing gas passing through the distributor plate is insufficient to break up the aggregates which form and the bed will not refluidize. These polymers are classified as those, which have a minimum bin opening for free flow at zero storage time of two feet and a minimum bin opening for free flow at storage times of greater than five minutes of 4 to 8 feet or more.
Sticky polymers can also be defined by their bulk flow properties. This is called the Flow Function. On a scale of zero to infinity, the Flow Function of free flowing materials such as dry sand is infinite. The Flow Function of free flowing polymers is about 4 to 10, while the Flow Function of non-free flowing or sticky polymers is about 1 to 3.
Although many variables influence the degree of stickiness of the resin, it is predominantly governed by the temperature and the crystalinity of the resin. Higher temperatures of the resin increase its stickiness while less crystalline product such as very low density polyethylene (VLDPE), ethylene/propylene monomer (EPM), ethylene/propylene diene monomer (EPDM) and polypropylene (PP) copolymers usually display a larger tendency to agglomerate in larger particles.
The mechanical agitation of a fluid bed or stirred gas phase reactor is sufficient to prevent the agglomeration of sticky polymers in the vessel. However current discharge systems utilized with gas phase reactors are not designed to process sticky polymers.
For example, U.S. Pat. No. 4,621,952 issued Nov. 11, 1986 discloses a product discharge system having an operation incompatible with the removal of sticky resins. In this system, the solid polymer is removed intermittently from the reactor into a holding vessel by means of a substantial pressure gradient. The resin particles impact the vessel wall with great momentum leading to the compaction and agglomeration of the sticky polymer. In addition, the depressurization phase necessary for the transfer of the resin to lower pressure process equipment requires a lead time during which the bed is motionless and subject to consolidation in a large chunk.
Another method and apparatus used to remove polymer resins from a fluid bed gas phase reactor is of the type disclosed in Japanese Disclosure number J 58032634-A. The apparatus comprises a long screw feeder connected to the gas phase fluid bed reactor via a feed pipe. The lead of the screw has a decreasing pitch, the purpose of which is to compact the solid polymer and increase its bulk density as the polymer is transported away from the reactor. The effect of the compaction is to increase the resistance of the flow from the high pressure region of the reactor to the low pressure region of the conveying line where the resin is discharged. Unfortunately the compaction of the polymer solid on which the successful operation of the apparatus is based is incompatible with the processing of sticky polymers. During compaction, the sticky polymer sinters in large chunks resulting in the failure of the screw or the blockage of the discharge end.
Another problem associated with the production of sticky polymers is the removal of monomer and volatile residues from the solid polymer. To avoid agglomeration of the particles in the gas phase reactor, a compromise exists between the temperature of the polymerization reaction and the degree of agitation required to maintain the bed in a free-flowing state. The production of sticky polymers requires substantially lower process temperatures in comparison to free flowing resins. The lower temperature of the solid polymer significantly reduces the desorption rates of volatile residues.
Purge bins which are commonly used in conjunction with gas phase reactors are presently unsatisfactory to devolatilize monomer residues of sticky and relatively cold polymers. A purge bin contains a bed of polymer resin slowly moving downward as a plug flow while being swept by an inert gas counter-currently. Compaction of the resin and low degree of inter-particle motion results in the sintering of the entire bed. U.S. Pat. No. 4,372,758 is an example of this process and is incorporated by reference.
Fluid bed purgers provide the necessary agitation of the sticky polymer but are impractical in treating cold solid polymers. The size of the fluid bed purger necessary to provide the required devolatilization time at low polymer temperatures and the amount of inert gas needed to devolatilize the monomer residues are economically prohibitive. Heating the resin is not practical either since higher temperatures would initiate resin particle aggregation leading to the defluidization of the bed.
The deactivation of Ziegler-Natta catalysts and aluminum alkyl cocatalyst residues embedded in the solid polymer is frequently carried out in parallel with the devolatilization process in the purge bin as demonstrated in U.S. Pat. No. 4,731,438 issued Mar. 15, 1988. The deactivating agent of choice is water although other compounds may be substituted. A stream of water vapor is added to the inert purge gas to form a mixture which percolates upward in the resin bed. The water then contacts the resin and neutralizes the catalyst and cocatalyst residues. Volatiles generated from the deactivation reaction are evolved in a manner similar to those dissolved in the solid polymer. However the poor handling properties of sticky resins and the impracticality of using the purge bin as previously discussed poses a problem in carrying out the deactivation of catalyst and cocatalyst residues.
The incorporation of additives in sticky resins is also problematic. Again the poor handling characteristics of the resin do not allow the practice of current methods based on dry blending or on the letdown of a granular masterbatch made of a concentrate of additives in free flowing resins.
Finally, solid polymer transport systems based on dilute phase conveying or dense phase conveying can not be utilized with sticky resins. In the dilute phase system, sticky resin particles tend to adhere to the walls, particularly in bends, leading to the rapid fouling of of the transfer line. The dense phase conveying system is also not suitable for the transport of sticky resins since the large compressive forces acting on the resin particles result in the formation of chunks leading to transfer system failure.